Amphetamine, a widely abused psychomotor stimulant drug, when taken over periods of days or weeks by human abusers may produce a condition termed "amphetamine psychosis" which, because of its remarkable similarity to paranoid schizophrenia, has been regarded as a valuable heuristic model for this significant mental disorder. Thus, understanding the mechanisms of action of amphetamine and related antipsychotic drugs may shed light on both amphetamine psychosis and, perhaps, paranoid schizophrenia. By means of local infusion at low concentrations and infusion volumes, or intraperitioneal injections of amphetamine and related compounds, significant changes in neuronal activity of neurons in the caudate-putamen and associated afferent projection systems will be sought. Similar experiments will be undertaken to determine how such alterations in neuronal activity following acute amphetamine administration are changed by long-term amphetamine administration for periods of from 1-5 weeks. Experiments will focus initially upon the nature of changes in neuronal activity following acute or long-term amphetamine administration, and later, changes in neuronal activity following intraperitoneal administration of direct-acting dopamine agonists, such as apomorphine, will be determined in order to test the hypothesis that post-synaptic receptors for dopamine are altered by chronic amphetamine use.